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Jun-Hong An

Publications and source records attributed to Jun-Hong An.

At least 19 recordsLinked to original sources
arXiv2026

Breakdown of Linearized Dispersion in Waveguide QED

Exploring interactions between quantum emitters and guided photons, waveguide quantum electrodynamics (wQED) offers a versatile platform for quantum simulation and quantum interconnect. To simplify the quantized light-matter interactions, a linearized dispersion approximation (LDA) was widely used in wQED under the conditions of weak coupling and narrow photonic bandwidth. However, a growing number of strong-coupling wQED experiments have called the validity of LDA into question. Here, by investigating giant atoms coupled to a semi-infinite waveguide, we propose a principle to judge the validity of LDA. We find that the decoherence of the atoms exhibits rich dynamical behaviors determined by different numbers of bound states in and out of the continuum (BICs and BOCs) formed in the energy spectrum of the total atom-photon system. Based on this, we show that despite the LDA can partially preserve some of BICs, it completely erases BOCs. This causes the dominant role of LDA in the atomic dynamics to break down. Providing a physical insight into the validity of LDA, our results lay a firm foundation for exploring fundamental physics and its application frontier in strong-coupling wQED platforms.

quant-ph↗
arXiv2026

Decoherence suppression in quantum dense coding

By harnessing entanglement to transmit information beyond the limits of classical communication, quantum dense coding serves as a key component in various quantum technologies. However, in the noisy intermediate-scale quantum era, noise-induced decoherence poses inevitable challenges to its practical implementation. By investigating the non-Markovian decoherence dynamics of the two categories of quantum dense coding protocols realized in discrete- and continuous-variable systems, we demonstrate a physical mechanism to suppress the decoherence in quantum dense coding. It is found that the formation of a bound state in the energy spectrum of the entire system consisting of each subsystem and its local environment of the quantum channel restores the quantum advantage of dense coding in the presence of decoherence. Refreshing our understanding on the noisy effect on quantum dense coding, our finding offers a guide toward the practical realization of noise-tolerant quantum dense coding.

quant-ph↗
arXiv2026

Axion detection in the non-Markovian noise

The detection of axion dark matter poses a significant challenge due to its exceptionally weak coupling to standard-model particles. Here we develop a noise-tolerant quantum-metrology scheme based on coupled $^{129}$Xe spin ensembles to search for such weakly interacting particles. Within an experimentally accessible parameter regime, our scheme achieves an ideal exclusion limit on the axion coupling constant that is several orders of magnitude more stringent than that of conventional detection methods. We further investigate the performance of our scheme under dissipative noise and find that its robustness is fundamentally governed by the energy-spectrum structure of the composite spin-environment system. Remarkably, when a bound state emerges in the spectrum, the exclusion limit asymptotically recovers its ideal value in the non-Markovian dynamics, whereas under the Markovian approximation the limit deteriorates significantly due to noise. Our results establish a universal mechanism for noise-resilient exploration of beyond-standard-model physics, which can be practically implemented by engineering bound-state formation via tuning the Larmor frequency of the spin ensembles.

quant-ph↗
arXiv2026

Non-Canonical Quantum Otto Engine

The fast-developing quantum technology is propelling thermal machine, which lies at the heart of thermodynamics, into a new golden era. Being stochastic because of the non-negligible thermal and quantum fluctuations, the efficiency and power of a finite-time quantum heat engine exhibit a trade-off, preventing the simultaneous achievement of high power and high efficiency. Conventional studies treat the statistics of a heat engine by assuming canonical thermalization, which is valid only under weak-coupling conditions. We here investigate the noncanonical effect on the statistics of the efficiency and power for a finite-time quantum Otto cycle. It is revealed that their average values and variances are highly controllable by engineering the energy spectrum structure of the composite system consisting of the working substance and the bath. Near the quantum critical point induced by the formation of a bound state in the energy spectrum, the average efficiency and power can be boosted simultaneously, meanwhile, both of their variances are decreased. Deepening our understanding of the usefulness of quantum reservoir engineering in thermodynamics, our result lays the foundation for the realization of efficient and stable quantum energy devices.

quant-ph↗
arXiv2026

Converting $PT$-Symmetric Topological Classes by Floquet Engineering

Going beyond the conventional classification rule of Altland-Zirnbauer symmetry classes, $PT$ symmetric topological phases are classified by $(PT)^2=1$ or $-1$. The interconversion between the two $PT$-symmetric topological classes is generally difficult due to the constraint of $(PT)^2$. Here, we propose a scheme to control and interconvert the $PT$-symmetric topological classes by Floquet engineering. We find that it is the breakdown of the $\mathbb{Z}_2$ gauge, induced by the $π$ phase difference between different hopping rates, by the periodic driving that leads to such an interconversion. Relaxing the system from the constraint of $(PT)^2$, rich exotic topological phases, e.g., the coexisting $PT$-symmetric first-order real Chern insulator and second-order topological insulators not only in different quasienergy gaps, but also in one single gap, are generated. In contrast to conventional Floquet topological phases, our result provides a way to realize exotic topological phases without changing symmetries. It enriches the family of topological phases and gives an insightful guidance for the development of multifunctional quantum devices.

cond-mat.mes-hall↗
arXiv2026

Floquet engineering in hybrid magnetic quantum systems

The advancement of magnonics has facilitated the utilization of hybrid magnetic systems in quantum technologies. A hybrid magnetic lattice formed by an array of superconducting loops and magnetic particles has been devised as a quantum bus to disseminate quantum resources among magnetic quantum entities serving as nodes of a quantum network. However, the lattice also exerts a decoherence effect on the quantum entities, which impairs its practical performance. By studying the non-Markovian dynamics of nitrogen-vacancy centers and magnon modes coupled to two independent hybrid magnetic lattices, we propose a Floquet-engineering scheme via periodic driving on the quantum entities to suppress decoherence. We find that significant steady-state entanglement is preserved when a Floquet bound state exists in the quasienergy spectrum of the system consisting of each driven quantum entity and its lattice. This result enables a precise manipulation of hybrid magnetic systems and benefits their applications in quantum networks.

quant-ph↗
arXiv2026

Protection of quantum steering ellipsoids in non-Markovian environments

Quantum steering ellipsoids (QSEs) provide a geometric representation, within the Bloch picture, of all possible states to which one qubit can be steered through measuring another correlated qubit. However, in realistic settings, quantum systems are inevitably coupled to their environment, resulting in decoherence and degradation of the QSE. Here, by investigating how local dissipative environments coupled to each qubit affect the quantum steering, we find that the geometry of each party's QSE is closely tied to the non-Markovian effect and the formation of a bound state in the energy spectrum of the total qubit-environment system. The bound state provides the ability and the non-Markovian effect provides the dynamical way for preserving the QSE. We systematically examine the characteristics of QSEs under three distinct scenarios, i.e., two-sided bound states, one-sided bound states, and no bound state, revealing a diverse range of steering types. Our work establishes quantum reservoir engineering as a tunable strategy for protecting and controlling quantum steering in open systems, offering a practical pathway toward robust steering-based quantum technologies.

quant-ph↗
arXiv2026

Hybrid-order nonlinear topological phases

The bulk-boundary correspondence (BBC), which relates topological invariants to boundary modes, is well understood for linear systems but remains an open question in the presence of nonlinearity, where multigap topologies make the BBC obscure and the topological description troublesome. We address this by developing an auxiliary-system formalism that enables topological classification of nonlinear-eigenvalue systems. In the two-dimension (2D) case, we show that two fixed eigenvalues can harbor first-order gapless boundary modes and second-order corner modes. Stacking this 2D system along the third dimension (3D) reveals distinct hybrid-order realizations. Under uniform stacking, the corner states in the $xy$ plane transform into hinge states along the $z$ axis, yielding a 3D second-order phase, while the gapless boundary states become side-surface states, yielding a 3D first-order phase. For dimerized stacking, these states are further confined to the two ends of the $z$ axis, yielding a 3D third-order phase, and localized at the hinges, yielding a 3D second-order phase. Our results establish a multiband bulk-boundary correspondence and identify stacking engineering as a versatile platform for exploring hybrid-order topological phases in nonlinear systems.

cond-mat.mes-hall↗
arXiv2026

Extreme volume monogamy via bound-state engineering

Quantum steering ellipsoid (QSE) provides a faithful representation of a two-qubit state. When extended to tripartite systems, the steerability from a trusted party to different receivers is subject to volume monogamy relations, which only constrain the total steerability but cannot individually eliminate the steerability of an untrusted third party, leaving a potential channel for information leakage via steering. Here, we show that this residual steerability can be completely suppressed by selectively engineering bound states in local qubit-environment subsystems, without compromising the steerability between trusted parties. Specifically, when bound states are formed in the subsystems formed by the trusted parties and their environments but absent in the untrusted one, the untrusted party's QSE volume decays to zero, while the trusted party's QSE volume remains finite. Our results establish selective bound-state engineering as a mechanism for extreme volume monogamy, with potential applications in secure quantum communication with an untrusted third party.

quant-ph↗
arXiv2026

Strongly coupled giant-atom waveguide quantum electrodynamics

Describing systems of superconducting atoms coupled to a continuum of photonic modes at multiple separated locations in a waveguide, waveguide quantum electrodynamics (QED) with giant atoms has emerged as a promising platform for realizing quantum interconnect. Such systems have been reported to exhibit rich phenomena that differ from those of natural atoms. Going beyond the widely used Born-Markov and Wigner-Weisskopf approximations, we investigate the non-Markovian dynamics of one and two giant atoms interacting with a waveguide formed by an array of coupled resonators. We discover that the diverse dynamical behaviors of the giant atoms are intrinsically determined by the energy spectrum of the composite system consisting of the giant atoms and the photonic modes in the waveguide. As long as one and more bound states are present in the energy spectrum, their excited-state probabilities, respectively, tend to stable finite values and lossless Rabi-like oscillations with frequencies proportional to the differences of the bound-state eigenenergies. Our result provides an insightful guideline for suppressing the decoherence of giant atoms and facilitates the development of quantum interconnect devices using giant-atom waveguide QED.

quant-ph↗
arXiv2026

Quantum battery optimized by parametric amplification

The parametric amplification enabled by two-photon driving constitutes a versatile platform for advanced quantum technologies. We present an optimized scheme for implementing quantum batteries (QBs) based on a superconducting circuit system, where a two-photon-driven LC resonator serves as the charger and an array of transmon qubits functions as the battery. Our results show that two-photon parametric driving exponentially enhances the effective cavity-qubit coupling, which in turn gives rise to near-degenerate energy-level structures and highly entangled quantum states. This significantly enhances the charging power and enables rapid energy transfer from the charger to the battery. Moreover, the engineered squeezed cavity mode and the associated quantum correlations effectively suppress environmentally induced decoherence, thereby delaying energy leakage and facilitating stable energy storage. The proposed scheme remains robust against practical experimental imperfections, such as parameter disorder and environmental noise, preserving its performance advantages. The work provides a feasible platform for realizing high-power, high-stability QBs and highlights the potential of parametric control in quantum energy technologies.

quant-ph↗
arXiv2026

Protecting spin squeezing from decoherence

As a crucial resource in the field of quantum metrology, spin squeezing can facilitate highly precise measurements that surpass the limitations imposed by classical physics. However, the quantum advantage of spin squeezing is significantly compromised by decoherence, thus impeding its practical implementation. Here, by investigating the influence of local dissipative environment on spin squeezing beyond the conventional Born-Markov approximation, we find a mechanism to protect spin squeezing from decoherence and show that robust spin squeezing can be achieved in the steady state. We outline an experimental proposal to verify our prediction in a trapped-ion platform. Overcoming the challenges set by decoherence in spin squeezing, our work provides guidance to realize high-precision sensing in realistic environments and sheds light on the effect of non-Markovian environment on quantum systems.

quant-ph↗
arXiv2026

Noise-Resilient Quantum Reinforcement Learning

As a branch of quantum machine learning, quantum reinforcement learning (QRL) aims to solve complex sequential decision-making problems more efficiently and effectively than its classical counterpart by exploiting quantum resources. However, in the noisy intermediate-scale quantum (NISQ) era, its realization is challenged by the ubiquitous noise-induced decoherence. Here, we propose a noise-resilient QRL scheme for a quantum eigensolver with a two-level system as an agent. By investigating the non-Markovian decoherence effect on the QRL for solving the eigenstates of the agent-environment interaction Hamiltonian, we find that the formation of a bound state in the energy spectrum of the total agent-noise system restores the QRL performance to that in the noiseless case. Providing a universal physical mechanism to suppress the decoherence effect on quantum machine learning, our result lays the foundation for designing NISQ algorithms and offers a guideline for their practical implementation.

quant-ph↗
arXiv2026

Quantum gravimetry with mechanical qubits

Levitated mesoscopic particles hold the promise of revolutionizing gravity sensing by using quantum effects. However, conventional quantum gravimeters based on such systems fail to harness the intrinsic large-mass advantage of the particles, because their commonly utilized auxiliary quantum systems counteract the role of mass as a resource. To overcome this limitation, we propose a quantum gravimetry by directly using the mechanical qubit (QM) formed by a levitated particle as the gravity sensor. Without resorting to the auxiliary quantum system, our scheme enables a straightforward readout of the particle's motion under gravitational influence. The obtained sensitivity behaves as a $m^{-1/2}$-scaling with the mass $m$. We also generalize our scheme to the \textit{mechanical cat qubit} as the gravity sensor. The sensitivity further scales as $N^{-1/2}$ with the mean phonon number $N$. In the experimentally realizable parameter regime, a sensitivity on the order of $0.1~ \text{\textmu}\text{Gal}/\sqrt{\text{Hz}}$ can be achieved, which outperforms the traditional schemes by two orders of magnitude. Reaching the \textit{double standard quantum limits} with $m$ and $N$ simultaneously, our scheme provides a feasible route toward compact high-sensitivity quantum gravimetry.

quant-ph↗
arXiv2026

Chiral quantum batteries

Exploiting quantum effects for energy storage, quantum batteries (QBs) offer compelling advantages over conventional ones in terms of superior energy density, ultrafast charging, and high conversion efficiency. However, their realization is hampered by decoherence, which causes incomplete charging, rapid self-discharging, and reduced extractable work. Here, we propose a QB architecture based on a chiral magnonic platform. It comprises two yttrium iron garnet (YIG) spheres, one serving as the charger and the other as the QB, coupled to a waveguide. The unique chiral coupling between magnons and the guided electromagnetic fields breaks inversion symmetry, inducing both nonreciprocal energy flow and coherent interference between the charger and QB. Their synergy endows our QB with a 34-fold increase in energy capacity and a 55-fold boost in extractable work compared to its achiral counterpart in an experimentally accessible regime. Our scheme harnesses the decoherence from the electromagnetic fields and turns its destruction into an asset, which enables the robustness and wireless-like remote charging features of the QB. Our analysis reveals that these extraordinary capabilities stem from quantum coherence. By establishing chirality as a useful quantum resource, our work paves a viable path toward the realization of QBs.

quant-ph↗
arXiv2026

Topological Phases in Non-Hermitian Nonlinear-Eigenvalue Systems

The discovery of topological phases has ushered in a new era of condensed matter physics and revealed a variety of natural and artificial materials. They obey the bulk-boundary correspondence (BBC), which guarantees the emergence of boundary states with nonzero topological invariants in the bulk. Widespread attention has been paid to extending topological phases to nonlinear and non-Hermitian systems. However, the BBC and topological invariants of non-Hermitian nonlinear systems remain largely unexplored. Here, we establish a complete BBC and topological characterization of the topological phases in a class of non-Hermitian nonlinear-eigenvalue systems by introducing an auxiliary system. We restore the BBC broken by non-Hermiticity via employing the generalized Brillouin zone on the auxiliary system. Remarkably, we discover that the interplay between non-Hermiticity and nonlinearity creates an exotic complex-band topological phase that coexists with the real-band topological phase. Our results enrich the family of nonlinear topological phases and lay a foundation for exploring novel topological physics in metamaterial systems.

cond-mat.other↗
arXiv2026

Quantum router of silicon-vacancy centers via a diamond waveguide

As a key component of quantum networks, the quantum router distributes quantum information among different quantum nodes. The silicon-vacancy (SiV) center in diamond offers a promising platform for quantum technology due to its strong strain-induced coupling with phonons. However, the development of a practical quantum router faces the challenges of achieving long-range entanglement and suppressing decoherence. Here, we propose a non-Markovian quantum router based on a diamond waveguide embedded with an array of SiV centers as the quantum nodes. Unlike conventional channel-switching methods, our design enables parallel quantum-state transfer from a single input node to multiple target nodes, analogous to a classical WiFi router. We demonstrate that persistent entanglement and suppressed decoherence of the SiV centers over long distances are achievable when bound states are present in the energy spectrum of the total system formed by the SiV centers and the phonon waveguide. Our scheme enriches the implementation of quantum routing and prompts the development of solid-state quantum networks.

quant-ph↗
arXiv2026

Breakdown of the symmetry constraint in a Floquet topological insulator

A topological insulator is regarded as an ideal candidate for information storage and high-speed lossless electrical transmission devices due to robust topological protected boundary modes. Previous studies revealed that symmetry exerts an unbreakable constraint on the existence, classes, and orders of its boundary modes. It severely limits the controllability and application of a topological insulator. Here, we propose a Floquet-engineering method to break this symmetry-imposed constraint on a topological insulator. By applying periodic driving on a system belonging to a symmetry class that prohibits the existence of first-order topological phases, we find that rich first-order boundary modes are created. Interestingly, exotic hybrid-order topological insulators with coexisting first-order helical boundary modes and second-order corner modes not only in two different quasienergy gaps but also in one single gap are generated easily by periodic driving. Refreshing the prevailing understanding of symmetry constraint on topological phases, our result opens an avenue for the creation of exotic topological insulators without altering symmetries. It greatly expands the scope of the fabricated materials that host topological insulators.

cond-mat.mes-hall↗